In the present research project, we have employed a combination of physiological, pharmacological and behavioral approaches to the study of somatosensory systems related to pain and hyperalgesia. The activity of lumbar spinal dorsal horn lamina I projection neurons was studied in normal rats and in rats with an inflamed hindpaw. Inflammation was produced by injecting Freund's adjuvant into the plantar surface 4 hours to 5 days prior to electrophysiological recording. Neurons recorded ipsilateral to the inflammation had enlarged and more complex receptive fields as compared to those observed in control rats. Many cells had high background firing rates including some burst activity and some cells responded to joint movement in addition to cutaneous stimulation. The increased activity of this population of neurons may result in a range of pain sensation. We have also begun to examine the central changes that occur during inflammation by characterizing the spinal cord pharmacology of nociception/antinoception in a behavioral model of hyperalgesia.